Sheathed electric heating unit



May 17, '1966 R. D. BAXTER sHEATHED ELECTRIC HEATING UNIT Original Filed Sept. 28, 1962 J E x V INVENTR v ROBERT D. BAXTER I .E M E v.

HIS ATTORNEY United States Patent Divided and this application Mar. 25, 1965, Ser. No.

5 Claims. (Cl. 3138-271) This application is a division of my copending application Serial Number 226,927; filed on September 28, 1962, now abandoned. The invention relates -to a sheathed electric heating unit and is particularly applicable to glow plugs which are commonly used in ignition systems such as, for example, in the ignition systems of diesel engines.

In the typical glow plug design, a heating element, usually in the form of a coil of resistance wire, is imbedded in compacted insulating material within an outer metallic sheath. One end of the heating coil is electrically connected to the tip end of the sheath, typically by welding the end of the coil to the sheath at that point, and the other end of the coil is electrically connected to a terminal which extends from the sheath at the end of the plug opposite the tip. l

It is important invsuch a unit from the standpoint of ensuring adequate operating life that the coil of 4resistance wire be properly centered in the sheath to prevent the development of a short circuit between the coil and the sheath, and also that the diameter of the wire be selected in light -of the other design parameters to avoid operation of the. coil at excessively high temperatures. In general, it is desirable that the diameter of the resistance wire be as large as possible within the physical confines of the design, taking into account, of course, the other design parameters involved. For a given heat dissipation rate, a larger wire diameter provides a greater wire surface area and therefore reduces the heat dissipation rate per unit surface area of the wire. This permits the attainment of a longer operating life for the unit.

In addition to -the foregoing factors, it is also important that theispacing between adjacent turns `of the heating coil be uniform along the length of the sheath, particularly near the tip end of Ithe'plug. Uneven spacing of the coil turns causes some portions of the unit to run hotter and others to run cooler than the desired temperature, although a slightly closer coil spacing may be desirable near the terminal end of a glow plug because in the typical installation some of the heat is usually conducted away from the plug through the terminal end. It is particularly important, however, to avoid excessive spacing near the tip of the plug because this causes the tip or working end of the plug to run cooler than the rest of the unit.

Heretofore in the manufacture of glow plugs it has been necessary to weld a coil lof resistance wire to one end of a sheath, attach a terminal to the opposite end of the coil and then stretch the coil longitudinally within the sheath to center it while granulated insulating material is loaded into the sheath. Stretching of the coil Vpermits it to be held in a center position in the ysheath and it also allows the granulated insulating material to How into the center of the coil through the resulting spaces between the coil turns.

The welding of the coill to the sheath, however, anneals and softens that end of the coil so that the above-mentioned stretching technique causes the coil to stretch by a greater amount near the tip end of the plug, thus producing a greater spacing between adjacent coil turns and causing the tip end of the plug to run cooler than the rest of the unit. Y In addition, the requirement for stretching reduces the diameter of the resistance wire which can be accommodated because the spaces between the adjacent coil turns produced by the stretching-represent spaces which could at least partially be occupied by wire of a greater diameter.

Attempts to avoid these problems by reducing the amount of tension applied to the terminal have, however, met with difficulties because the problem of centering the coil in sheath becomes more acute and the reduced spacing between the adjacent coil turns impedes the flow 0f granulated insulating material into the center of the coil. Attempts to compromise between the foregoing factors have produced diiculties with respect to cooler than desired operating temperatures near the tip of the plug, adequate centering of the coil in the sheath and the attainment of proper operating life through the use of optimum resistance wire diameters.

Accordingly, it is an object of this invention to provide an improved sheathed heater of the glow plug type having an increased operating life.

It is another object of this invention to provide an improved sheathed heater of the glow plug type having a more uniform spacing between coil turns, particularly near the tip end of the unit, while retaining accurate centering of the coil within the sheath.

Briefly stated, these and other objects of this invention are accomplished, in'one embodiment thereof, in an improved sheathed heater wherein a frangible tube of solid insulating material is utilized between the outer diameter of a heating coil and the inner wall of a sheath to center the coil in the sheath. The terminal arrangement of the improved sheathed heater permits granulated insulating material to be loaded into the center. of the coil through a duct, which in one embodiment extends through a hollow terminal which is connected to the coil. The frangible tube of insulating material is then crushed and compacted along with the granular insulating material in the final compression of the unit.

Because stretching of the coil is not required, a more uniform spacing between the coil turns, particularly near the tip end of the unit, is obtained. In addition, since the granulated insulating material is not required to flow between the adjacent coil turns into lthe center of the coil, a closely wound coil may be used, thus permitting the utilization of a maximum wire diameter.

For a thorough understanding of this invention, reference may be had to 'the following description, taken in connection with the accompanying. drawing in which: FIG. 1 is a cross-sectional view of a glow plug, at an intermediate stage of manufacture, embodying this invention and illustrating the initial steps of the method of this invention.

FIG. 2 is a cross-sectional view of the glow plug of FIG. l at a subsequent stage of manufacture and during the insulation loading operation.

FIG. 3 shows the glow plugs of FIGS. l and 2 in its completed form.

FIG. 4 shows, in vertical cross-section, a glow plug similar to that of FIGS. l-3 but embodying a modified terminal arrangement.

to the process step in which powdered insulating material is loaded int-o the unit. At this stage of the manufacture, 4the glow plug assembly 10 comprises an outer sheath 14, normally constructed of metal or other electrically conductive material, in which is disposed a heater assembly shown generally at 15. The heater assembly 15 comprises a heating element in the form of a coil 16 of resistance wire and a terminal 13 which is attached to one end 16a of the coil 16. A tube 18 of frangible insulating material is slidably mounted between the inner wall of the sheath 14 and the outer diameter of the coil 16 to center the coil in the sheath. The `tube 18 may be of any suitable insulating material, such as magnesium oxide.

A duct 12 extends through the terminal 13 .and is shown in FIGS. 1-3 in the preferred f-orm of a passagewayextending through the center of the terminal 13. In other words, the terminal 13 is hollow or tubular. The duct 12 provides a communicating passageway between the exterior of the coil 16 and its central or core portion through one end of the coil and, as will be ex plained later on in greater detail, the duct 12 may take other forms than the particular one illustrated.

To reach the point in the `assembly shown in FIG. 1, the terminal 13 is attached to the upper end of the coil 16a by welding or in any other suitable manner and the assembly thus formed is then inserted in the open end of the sheath 14. -The sheath is crimped as shown at 19 to hold the lowerend 16h of the coil tight and flush with respect to the end 14b of the sheath. The tube 18 of insulating material is placed in the sheath as shown in FIG. 1 to cen-ter the coil in the sheath. The lower end of the tube 18 bears against the reduced diameter portion of the crimped section 19 of the sheath.

At this point in the process, 'the lower end 14b of the sheath is welded closed, simultaneously joining the end 16h of the coil to the sheath so as to make an electrical connection therebetween as can be seen in FIG. 2. The steps of connecting the lower end 16b of the coil to the sheath and of closingI the lower end of the sheath may of course be accomplished in any suitable fashion ralthough I have found it convenient to accomplish both of these steps with 'a single welding operation as described above.

With the coil 16 -remaining in its normal unextended position, accurately centered by the tube 18 of compressed insulating material, .powdered insulating material 20 is introduced into the interior of the coil 16 through the top or open end of the duct 12 in the terminal 13. For satisfactory loading and subsequent compacting, the heat- Aing unit is preferably vibrated during the loading process.

Any suitable powdered material may be used, magnesium oxide being very satisfactory. lt has been found desirable to till the interior of the duct 12 with powdered insulation also since this prevents a void through which the powdered insulating material could otherwise flow from the center of the coil during subsequent swaging and compression of the sheath. The space at the top of -the coil between the terminal 13 and the tube of insulating material 18 is also lled with powdered insulat-v ing material.

'itwill be appreciated that with the duct 12inthe form shown in FIGS. 1-3, the loading of the powdered insulating material l'through the duct 12 may be accomplished with the disk 21 in place. With this latter approach, the disk 21 may be utilized to center the terminal 13 during the loading operation.

To complete the production of the heating unit, the upper end of the sheath is bent at 22, as shown in FIG. 3, to hold the disk 21 rmly in place and the upper portion 23 ofthe terminal 13 is crimped or plugged to prevent loss of insulating material from the unit. The unit is then swaged or otherwise compressed down to its final dimensions.

The final swaging or compression operation crushes the tube of insulating material 18 and compacts this crushed insulation along with the powdered insulating material which has been loaded into the sheath. In the production of the heater shown in FIG. 3, the diameter of the unit is reduced to two different dimensions. The portion 10a which includes the coil 16 is reduced to a smal-ler diameter than the remaining portion 10b. It should be noted at this point that the hollow terminal 13 provides a secondary advantage in that the 4terminal is squeezed down to a smaller diameter along with the coil 16 to provide 'additional radial clearance between the upper end of the coil and the sheath than would otherwise be available with a solid terminal where no similar reduction in diameter would occur. In the past it has been found necessary in some designs to utilize a smaller diameter wire around the terminal in order to avoid the problem of reduced radial clearance between the coil and unit. This closer spacing at the coil-terminal joint has been found to be advantageous in that it increases the density of heat generation at this end of the unit Where in the typical installation some of the heat is usually conducted away from the plug through the terminal end.

FIGS. 4 through 7 reveal two modified terminals providing ducts through which powdered insulating material may be introduced into the interior of the heating coil unit. In FIG. 4 a solid rod-type .terminal 33 having a duct or keyway 34 formed therein is shown affixed to one end of a coil 36. This modification is more clearly shown in the coil end View of the terminal 33 in FIG. 5. The slot 34 forms a passageway communicating With the interior of the coil 36 through the upper end of the coil and through which powdered insulating material may be loaded into the center of the coil.

Another alternative arrangement is shown in FIGS. 6 and 7. In this embodiment Ithe coil end of a rod-type terminal 43 is provided with a duct or slot 44 which forms a passageway through which insulating material may flow into .the linterior of the heating coil.

It should be noted that in the embodiments of FIGS. 4 through 6 the ducts which communicate with the center of the coil do not extend externally from the sheath and, accordingly, need not be externally closed off prior to the iinal swaging operation. On the other hand, with this type of duct structure, the loading of the insulating material into the center of the coil must be accomplished before the open end of the sheath is closed olf. Such alternative duct arrangements may be extended, of course, beyond the open end of the sheath, but the extended duct portion should then be cl-osed off or suitably plugged Ibefore the final swaging operation as is done in the case ofthe embodiment shown in FIGS. 1-3.

It will be observed that the foregoing method avoids the necessity for stretching the heating coil to center it in the shea-th during the loading operation. Accordingly, the problem of excessive stretching of the coil near the welded end is avoided. In addition, it will be noted that with the use of duct means communicating with the center of the coil through one end thereof to' accommodate -the loading of the powdered insulating material into the center of the coil, the need for stretching the coil to permit the insulating material to flow between adjacent turns of the coil is also eliminated. Such a technique therefore permits the use of a closely wound coil with the turns of the coil being closely adjacent or even touching each yother in the uns-tretched state, thereby allowing the use of a maximum resistance wire diameter. In other words, the use of a space 4wound coil, that is, a coil in which the turns are spaced relatively far apart in the unstretched state t-o permit insulating material to flow into the center of lthe coil through the spaces between lthe turns, is not required.

It will thus be observed that my invention produces an improved heating unit in |which the heating coil is accurately centered within the sheath with .the adjacent turns of the heating coil being more uniformly spaced and with a greater amount of heating wire being accommodated in the unit. The foregoing advantages take the form of a heating unit of substantially improved quality with a significantly increased operating life capability.

It will be appreciated, of course, that various modications may 'be made in the structures and in the process steps which I have described. For example, the tube 18 `of insulating material may be made up of a number of separate elements and may be in a diiferent form than the particular one which I have illustrated. In addition, various alternative methods of securing the terminal to the heating coil and of closing the ends of the sheath may be employed. The duct which communicates with the center of the coil to accommodate the loading of the powdered insulating material into that area may, ofl

course, take Various forms and I have illustrated several alternative approaches in this regard. Another approach would be to provide duct means into the coal through one end lthereof independently of the terminal structure,

although I prefer to combine the duct with the terminal.

Thus, -while I have described several embodiments of my invention in considerable detail it should be apprec-iated that I have done this for purposes of presenting a full and clear description and that my invention is not limited to the particular details which I have presented. Accordingly, it is to be understood that various changes, modifications and substitutions may be made in the subject matter presented herein without departing from the true scope and spirit of my invention as I have defined it in the appending claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A sheathed heating unit comprising:

(a) an electrically conductive sheath having a closed end,

(b) a heating element of generally hollow open-ended form substantially centrally spaced from the inner wall of said sheath and electrically connected to said closed end of said sheath,

(c) insulating material including at least one compressed tubular frangible member, said tubular.

frangible member positioned prior to compression between the inner wall of said sheath and the outer surface of said heating element to maintain said heating element with respect to said sheath, said in- 2. A sheathed heating unit comprising:

(a) an electrically conductive sheath having a closed end,

(b) a heating coil substantially centrally spaced from the inner wall of said sheath and electrically connected to said closed end of said sheath,

(c) a mass of insulating material in which said coil is imbedded within said sheath, and v (d) a substantially solid terminal having one end electrically connected to said coil and being relatively centrally disposed in said sheath, said terminal having a slotted passageway formed in said one end which communicates with the interior of said coil during manufacture, whereby insulating material may be introduced into the interior of the coil.

3. A sheathed heating unit comprising:

(a) an electrically conductive sheath having a closed end,

(b) a heating coil substantially centrally spaced from the inner wall of said sheath and electrically connected at one end to said closed end of said sheath,

(c) insulating material including a compressed tubular frangible member, said tubular frangible member positioned prior to compression between the inner wall of said sheath and the outer surface of said heating coil to maintain said heating coil with respect to said sheath, said insulating material also including powdered insulation within the interior of said heating element, and

(d) a solid cylindrical terminal having one end electrically connected to the other end of said coil and being substantially centrally disposed in said sheath, said terminal having a passageway from the side of said terminal through said one end, which communicates with the interior of the coil during manufacture whereby said powdered insulation may be introduced into the interior of the coil.

4. A sheathed heating unit comprising:

(a) an electrically conductive sheath having a closed end,

(b) a heating element of generally hollow open-ended form substantially centrally spaced from the inner wall of said sheath and electrically connected to said closed end of saidsheath,

(c) insulating material including a compressed tubular frangible member, said tubular frangible member positioned prior to compression between the inner wall of said sheath and the outer surface of said heating element to maintain said heating element with respect to saidrsheath, said insulating material also including granular insulation within the interior of said heating element, and

(d) to hollow terminal connected at one end to said element and centrally disposed in said sheath, said one end of said terminal being in communication with the interior of said element during manfacture whereby said granular insulation may be introduced into the interior of said element through'said terminal.

5. A sheathed heating unit comprising:

(a) an electrically conductive sheath having a closed end,

(b) a heating element of generally hollow open-ended form substantially centrally spaced from thev inner surface of said sheath and electrically connected to said closed end of said sheath,

(c) at least one compressed tubular frangible member of insulating material, said tubular fran-gible member positioned prior to compression between the inner wall of said sheath and the outer surface of said heating element to maintain said heating element with respect to said sheath, and

' (d) a solid elongated terminal having one end electrically connected to said element and being substantially centrally disposed in said sheath, said terminal having a passageway from a side ofvsaid terminal 2,506,768 5/ 1950 Beutz et al 123-145X through said one end, which communicates with the 2,884,920 5/ 1959 Moule et al '123-145 4 interior of said element during manufacture whereby 2,898,571 8/1959 Moule et al 338,--238 powdered insulating material may be introduced into 3,158,787 11/ 1964 Testerini 317--98 the iuterlor of sa1d element. 5 FOREIGN PATENTS References Cited by the Examiner 461,936 2/ 1937 Great Britain.

UNITED STATES PATENTS 493,331 10/ 1938 Great Britain. 1,394,937 10/ 1921 Reichold 338-241 RICHARD M. WOOD, Primary Examiner.

2,428,900 10/1947 Wiegand 29 .155.64 10 2,455,186 11/ 1948 McCormick 29--15564 V- Y- MAYEWSKY,AsszstantExammer.

UNITED STATES PATENT oFFIcE CERTIFICATE 0F CORRECTION Patent No 3 ,252 ,122 May 17 196| Robert D. Baxter It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 1, line 12, strike out "now abandoned" and insert instead now United States Patent No. 3,234,633

Signed and sealed this 1st day of August 1967.

(SEAL) Attest:

EDWARD I. BRENNE] Commissioner of Patents EDWARD M. FLETCHER, JR.

Attesting Officer 

1. A SHEATHED HEATING UNIT COMPRISING: (A) AN ELECTRICALLY CONDUCTIVE SHEATH HAVING A CLOSED END, (B) A HEATING ELEMENT OF GENERALLY HOLLOW OPEN-ENDED FORM SUBSTANTIALLY CENTRALLY SPACED FROM THE INNER WALL OF SAID SHEATH AND ELECTRICALLY CONNECTED TO SAID CLOSED END OF SAID SHEATH, (C) INSULATING MATERIAL INCLUDING AT LEAST ONE COMPRESSED TUBULAR FRANGIBLE MEMBER, SAID TUBULAR FRANGIBLE MEMBER POSITIONED PRIOR TO COMPRESSION BETWEEN THE INNER WALL OF SAID SHEATH AND THE OTHER SURFACE OF SAID HEATING ELEMENT TO MAINTAIN SAID HEATING ELEMENT WITH RESPECT TO SAID SHEATH, SAID INSULTING MATERIAL ALSO INCLUDING GRANULAR INSULATING WITHIN THE INTERIOR OF SAID HEATING ELEMENT, AND (D) A CYLINDRICAL, SUBSTANTIALLY SOLID TERMINAL HAVING ONE END ELECTRICALLY CONNECTED TO SAID HEATING ELEMENT AND BEING RELATIVELY CENTRALLY DISPOSED IN SAID SHEATH, SAID TERMINAL HAVING A SLOTTED DUCT FORMED IN SAID ONE END WHICH COMMUNICATES WITH THE INTERIOR OF SAID HEATING ELEMENT DURING MANUFACTURE, WHEREBY SAID GRANULAR INSULATION MAY BE INTRODUCED INTO THE INTERIOR OF SAID HEATING ELEMENT. 